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Energize Prevention of bearing currents in large inverter driven electrical...
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Prevention of bearing currents in large inverter driven electrical machines

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While bearing currents have been around since the advent of electric motors, the incidence of damage they cause has increased with the advent of high switching frequency variable frequency drives (VFDs). Inverters based on fast switching power electronics, such as IGBTs, allow improved operation of variable speed drives, but high switching speed leads to fast rising voltage pulses , that can cause inverter induced bearing currents. These bearing currents may destroy bearings within a short time of operation.

The problem of bearing currents in line frequency driven machines has been known for decades and has largely been solved. Early VFDs used a relatively low switching frequency, but current t VFD technology, incorporating insulated gate bipolar transistors (IGBT), creates switching events many times faster than those considered typical in the past. This has led to a rising number of electrical discharge machining (EDM) type bearing failures in AC drive systems, some relatively soon after startup.

Cause of  EDM current damage

Damage is caused by arcing between the race and the balls of the bearing due to the breakdown of insulation. Insulation is provided by means of the lubrication grease film. When a sufficiently high voltage is reached across the lubricating film,EDM currents flow on the contact surfaces of the bearing and erode the surface. The contact between the balls and bearing races is intermittent, caused by variations in the lubricant film. This results in the formation of arcs that, in turn, leads to EDM. Capacitive currents flowing across the bearing do not cause EDM. Both capacitive discharge currents and high frequency circulating currents damage the bearings.

Fig. 1: Common mode voltage waveform (ABB).

Cause of high frequency bearing currents

High frequency bearing currents are a result of current flow in the common mode circuit of the VFD. Common mode currents are the result of the common mode voltage (CMV) and motor capacitance.

Common mode voltage

The common mode voltage (CMV) is the voltage between the common or neutral point of a three phase system and earth. For a balanced sinusoidal wave system the CMV is zero. However, this is not the case with a PWM switched three-phase power supply, where a DC voltage is converted into three phase voltages. Even though the fundamental frequency components of the output voltages are symmetrical and balanced, it is impossible to make the sum of three output voltages instantaneously equal to zero with two possible output levels available. In a PWM VFD system the phase voltage consists of a series of pulses of varying duration but of equal voltage level, and CMV has a pulsed waveform as shown in Fig. 1. The voltage can be equal to the voltage of the pulsed phase waveform.

Voltage overshoot

The waveform shown in Fig. 1 exists at the output of the VFD. Once the motor and cable are connected, a further effect arises, that of voltage pulse overshoot. This causes the voltage at the edge of the pulse to rise above the pulse voltage for a short period ,and is mainly due to the inductance of the cable . Fig.2 shows typical voltage overshoot, which is reflected in the CMV and is a result of the combination of inverter, cable and motor characteristics.

Fig. 2: Voltage pulse overshoot (Schaffner).

The steep rise and fall times of the CMV cause high frequency current to flow through the motor capacitances to earth. Voltage overshoot compounds the problem by adding high voltage spikes to the CMV which increases the chance of arcing over the bearing.

Motor capacitances

Fig. 3 shows the various capacitances in an AC motor that are relevant when the motor is driven by a VFD. The high dv/dt of the common mode voltage applied across the stator and grounded frame of the motor causes pulsed currents to flow through the capacitances shown in Fig. 3.

Capacitances shown are:

  • Stator to frame capacitance (CSF): This is the capacitance that is formed between the stator winding and the earthed frame. Most of the common mode current due to the high dv/dt of the common mode voltage flows through this path.
  • Stator to rotor capacitance (CSR): This capacitance is formed  between the stator winding and the rotor frame. The value of this capacitance is rather small but is the principal path that charges the rotor body to which the motor shaft is physically connected, and the magnitude of the shaft voltage is dependant on the value of this capacitance.
  • Rotor to frame capacitance (CRF):  The value of this capacitance is typically about ten times that of the stator winding to rotor surface capacitance (CSR). Since the voltage across a capacitor is inversely proportional to its capacitance value, the most of the applied CMV appears across CSR and only a small voltage is developed across CRF or the rotor to frame structure.
  • Shaft to frame capacitance or bearing capacitance (CB): This capacitance is transient and is formed only when the motor rotates. When the motor is stationary or rotating at low speed there is metallic contact across the bearing. When the motor is rotated at or above a certain speed, the balls in a ball-bearing or rollers in a roller-bearing of the motor float and occupy the space in between the inner and outer race of the bearing. An insulating film is formed by the lubricant medium. The value of this capacitance depends on the shaft speed, type of lubricant used, the surface area of the ball or roller in the bearing, the temperature of the lubricant, and the mechanical load on the shaft. The value of this capacitance is important because its characteristics determine bearing current and dictates the life of the bearing.

Fig. 3: Motor inherent capacitance (Yaskawa).

Common mode capacitive currents

These comprise circulating and non-circulating currents.

Capacitive charge/discharge currents (non circulating currents)

Non circulating currents flow through the machine capacitance to earth. The steep rising and falling edges of the CMV give rise to charging/discharging current through the capacitance of the machine. The voltage across the bearing will depend on the ratio of the capacitances of the machine. Fig. 4 shows the capacitive paths within the machine that determine the bearing voltage.

Fig. 4: Capacitive current paths within the motor (Yaskawa).

If the voltage across the bearing reaches the breakdown voltage, current will flow until the voltage drops and the arc extinguishes, to fire again on the next cycle. The current flowing through the bearing to earth will take the form of pulses, rather than a continuous current flow. In a PWM VFD the size and duration of the current pulse will depend on the width of the PWM pulse, and will vary throughout the cycle. Typical current flow paths for currents are shown in Fig. 6.

  • Capacitive bearing currents (i1):  High CMV voltage dv/dt in the stator windings causes pulse currents to flow to the rotor through (CSR). These currents get distributed to two different paths. The first path is the return path formed by CRF and the second path is that formed by the bearing capacitance CB.
  • EDM bearing currents (i2): The CMV causes an electric charge to be stored in CRF. The voltage across this capacitor can keep building upand eventually reach such a level so as to cause the insulation of the lubricating film to breakdown. This dielectric breakdown causes CRF to discharge through the insulating film of the bearing creating what is known as EDM bearing current. Since the capacitance of CRF is relatively higher than CSR, the energy stored in CRF can be sufficiently large to cause bearing damage.
  • Common mode current flow through shaft due to poor earthing (i3): If the motor frame is poorly earthed and the motor shaft is connected to a mechanical load that has much lower earth impedance, the common mode current flows through CSR and charges up the rotor structure, now finds a way to flow through the shaft into an external earth that has a lower impedance.

Fig. 5: Bearing currents in a VFD driven motor (Yaskawa).

High frequency circulating currents

Circulating currents circulate around the frame of the motor via the shaft, and through the bearing capacitance, and are caused by a voltage developed across the shaft of the motor. This shaft voltage is caused by a high frequency flux field which develops in the core of the stator due to capacitive currents flowing in the stator winding. This flux filed couples inductively with the shaft ad develops a voltage in the shaft. The motor can, in this case, be thought of as a transformer, where the common mode current flowing in the stator frame acts as a primary and induces the circulating current into the rotor circuit or secondary (Fig. 6).

The frequency range for these circulating currents is in the kHz or MHz range. The size of these currents, and the damage they do, depends on motor size.  They first become a problem in motors above 75 kW, and in general, the larger the motor, the greater the damage they cause (Baldour [7]).

In Fig. 6 the circulating current is shown as (i4) The circulating current flows along the axis of the rotor, through the bearings and circulates through the stator frame and returns back from the other bearing end.

Fig. 6: Equivalent circuit  of rotor shaft coupling (ABB).

Influence of motor size

A study by Mutze [2] found a significant difference between the parasitic currents in large and small motors. Very small motors in the 1 kW range show small capacitive common mode currents and larger EDM-currents over the whole speed and temperature range. With larger motors (110 kW range) both circulating and EDM-bearing currents occur, depending on motor speed n and bearing temperature, with the circulating currents dominating. With very large motors (500 kW) EDM-bearing currents  are dominated by  distinctive circulating bearing currents.  Aegis [5] agrees somewhat with these findings, concluding that the high frequency circulating currents begin to dominate at motor sizes above 100 kW (Fig. 7).

Mitigation of parasitic currents

Parasitic currents are the result of the combination of motor, VF drive and connected load characteristics. Although all components are designed and manufactured to perform to specifications, the combination of different components can produce different effects and the specific combination, including connection cables, needs to be taken into account when deciding on mitigation methods. There are three approaches used to mitigate high frequency bearing currents: a proper cabling and earthing system; breaking the bearing current loops; and damping the high frequency common mode current.

Fig. 7: Parasitic currents in large motors (Aegis[6]).
Solutions can be classified in two groups:

  • Solutions applied on inverter side, and techniques to mitigate bearing currents within the motor.

The purpose is to reduce or eliminate the common mode voltage of the inverter, as it is the source of HF bearing currents. The first group comprises multi-level inverters, inverter output filters (dV/dt-reactors, dV/dt-filters), sinusoidal filters, common mode chokes and shielded cables. Common-mode filters can also be used to reduce or eliminate the CMV.

  • The second group includes HF bonding straps, rings, insulated bearings, ceramic or hybrid bearings, insulated couplings, or electrostatically shielded rotor.

Multi-level converters

As the converter of the VFD is the primary cause of the problem one of the first points is to consider the VFD itself.  2-level high frequency pulse width inverters for motor drives have problems associated with the high switching rate which produces CMV and high voltage change (dV/dt) rates to the motor windings. Multilevel inverters solve these problems by using a much lower frequency switching rate, and smaller voltage steps. Advanced inverter designs can reduce the common mode voltage to a very low value.

Shielded cables

Shielded motor cables provide a short, low impedance path for common mode current to return to the inverter. The shield must be continuous and of good conducting material, i.e., copper or aluminium. Using as short a length of cable as possible reduces the inductance and hence voltage overshoot. The VFD should be placed as close as possible to the motor.

Fig. 8: An example of reduction of overvoltage with a dv/dt filter (Schaffner).

Filters and chokes

A choke is an inductance fitted between the VFD and the motor. The choke consists of an inductive loop placed around the cables.

There are two basic types of motor filters: Sinewave and dV/dt.  Both are enhanced versions of chokes, adding more filter stages and other enhancements. The sinewave filter  is a low-pass LC filter in each phase of the motor which converts PWM into the corresponding sinewave with approximately the same RMS voltage as the original PWM signal. Sinewave filter offers advantages of greatly reduced EMI in all aspects. They also can be retrofitted in existing installations.  They cannot be used at lower switched frequencies due to possible internal capacitor damage, and are bulky.

dV/dt filters , which consist of cored coils inserted in the cable feed, “stretch” the rise and fall times of the drive pulses, reducing the high-frequency content of the drive signal which, in turn, reduces capacitive currents  through the bearings.  Can be installed “after the fact.”

Bearing protection rings (shaft grounding)

EDM and circulating currents are provided with a low impedance path that bypasses the bearing and connects the shaft to the motor frame. Bearing rings can be fitted internally or externally to the motor.

Insulated bearings

Insulated bearings the outer race is fitted with a insulating shield to prevent current flowing from the frame to the shaft through the bearing. (Fig.7).  Ceramic bearings break the current paths, while the insulating layers of insulated bearings reduce circulating bearing currents and bearing currents due to rotor ground currents,

Electrostatic shielding

Gerber [1], de Busse [6] and Baldour [7] found that electrostatic shielding of the motor windings can be effective in reducing inverter induced currents. Shielding creates  a grounded conductive path between the stator and the rotor that will bleed off capacitive coupled current. While this technology eliminates stator to rotor coupling, it does not eliminate the potential of coupling from the stator winding to the frame and through the bearing.

References

[1] S Gerber: “Reduction of inverter-induced shaft voltages using electrostatic shielding”, 27th SAUPEC, January 2019.
[2] A Mutze: “Bearing currents in inverter-fed AC-motors”, Dr-Ing. dissertation, TUD.
[3] ABB Technical guide No. 5: “Bearing currents in modern AC drive systems”.
[4] Yaskawa application note: “Motor Bearing Current Phenomenon and 3-Level Inverter Technology”.
[5] Aegis: “How do VFDs cause bearing damage”, Electro-Static Technology.
[6] D Busse, et al: “An Evaluation of the Electrostatic Shielded Induction Motor: A Solution for Rotor Shaft Voltage Buildup and Bearing Current”, IEEE IAS Conference 1996.
[7] Baldour Electric: “Inverter-Driven Induction Motors Shaft and Bearing Current Solutions”, white paper.

[8] Schaffner application note: “Sine wave filter solutions for motor drive applications”.

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